Extinction's group theory

June 25, 2004 — 10.00am

For more than three decades it has been known as the Blitzkrieg theory of extinction - the orgy of hunting and blood lust that took place when humans first arrived in new lands around the planet. Wonderful and bizarre giant beasts, known as megafauna, were rapidly pushed to extinction over the last 50,000 years and ancient man was the culprit.

By the time the killing was over some of the most impressive creatures that evolution has ever thrown up, such as the six-tonne woolly mammoth and the 2.7-tonne Australian diprotodon, had been wiped out from millions of square kilometres of their habitat.

The Americas, Australia, Madagascar, New Zealand and countless smaller islands were all supposedly overrun by hunters, leaving a trail of megafauna skeletons in their wake.

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Out of dozens of giant Australian species, the only undisputed megafauna mammal that survived the arrival of Aboriginal people is the red kangaroo.

The ecological carnage wrought by humans in this era of first contact between naive new-world wildlife and the all-conquering "cave" man is an indictment on our forebears.

The creatures were simply there for the taking - great slabs of protein used to fuel the invasion of entire continents and islands. The theory is both a testament to humanity's terrible omnipotence and our stupidity at not stopping until the last beast fell.

The only problem with this grisly story is that, to some of the researchers who study these ancient extinctions, it is becoming clear that the crime could not have been committed by human hunting alone.

There is no doubt that prehistoric hunters pursued big game. However, the truth about extinctions is possibly a lot less gruesome.

An increasing body of scientific literature and, most recently in Australia, a new scientific paper published in Alcheringa (the journal of the Australasian Association of Palaeontologists) say that the Blitzkrieg proponents have an overinflated view of early man's hunting prowess.

The Dean of Science at the University of NSW, Professor Mike Archer, has closely followed the debate and says the new paper by the University of Sydney scientists Dr Steve Wroe and colleagues Judith Field, Richard Fullagar and Lars Jermiin is another nail in the coffin for Blitzkrieg. It is also a rehabilitation of the reputation of the first inhabitants of the continent.

"Blitzkrieg was put forward in times when it was popular for people to be blamed for everything that was wrong in the world," Archer says. "Blame cannot be laid at the feet of humans based on any evidence that has turned up to date."

Australia's main proponent of Blitzkrieg, the director of the South Australian Museum, Dr Tim Flannery, disagrees. He cites other new research which says that when computer models are run - Blitzkrieg is inevitable. "The [new] paper presents a far from convincing case," Flannery says.

As anybody who has come face to face with a wild bull or a rampaging elephant will attest, they are mammal monsters that only a fool or someone very well armed would attack for fun.

Blitzkrieg depends on megafauna that are unaware of the danger that humans pose and therefore make relatively easy prey - such as island-based animals.

But Wroe and Field argue that to make parallels between island creatures and those on continents is ludicrous because, even in the absence of humans, Australia was home to a suite of deadly meat eaters. It has become apparent that one of these, Thylacoleo carnifex, the marsupial lion, represented the most extreme example of adaptation to a carnivorous lifestyle ever attained by mammals.

There is no way, argues Wroe, that a giant diprotodon would have been able to survive in the same ecosystem as the marsupial lion unless it had some serious chutzpah.

Not only did the herbivorous Australian megafauna have to cope with Thylacoleo, they had to guard against attacks from a medium-dog-sized Tasmanian devil, Tasmanian tigers, a giant goanna and meat-eating thunderbirds, which were dubbed by palaeontologists the "demon ducks of doom". All these carnivores would have made for creatures able to put up a fight against newly-arrived humans, the new paper argues.

Opponents of the Blitzkrieg theory say the real ringleader in the extinction of megafauna is climate change. In Australia the swings in aridity and temperature were extreme enough to turn relatively productive Central Australian habitats into horizon after horizon of red sand dune. Whatever humans did or didn't do, it was against a backdrop of climatic disintegration, says Wroe and his colleagues.

Also, stone spear points and the woomera (spear-thrower) do not appear in the Australian record until about 5000 years ago. The woomera in particular is a crucial big game hunting innovation as a spear thrown with one releases four times the kinetic energy of a modern compound bow.

But Flannery disagrees. "In places like New Zealand there's no hunting tool kits and yet they laid waste to the moa. What sort of tools would you need if these animals didn't recognise us as a threat?"

In an era when loss of biodiversity is one of the greatest issues facing the planet it is crucial for scientists to fully appreciate all of the subtleties of the extinction crisis.

Maybe the biggest lesson of all is that we flatter ourselves if we think we can explain at all the reasons for life and death.